Transcript What is the refractive index of Sodium Chloride (NaCl)?

Light Refraction as a Forensic Tool
PWISTA 12/2/2006
Objectives:
Matching Glass Fragments
Theory of Refraction
– Speed
– Angular
Refractometry
– Different Refractometers
Jell-O (Refractive Index) Lab Demo
Immersion Method of Glass Identification
– Becke Lines
Unknown lab
Matching Glass Fragments
Suspect and crime scene fragments must fit
together to be from same source
Physical properties of density and refractive index
are used most successfully for characterizing
glass particles.
1. Flotation test in density column!!
2. Immersion Method
3. GRIM 3: Glass RI measurement (automated)
1. Flotation test in density column
Control glass added to liquid
Density of liquid adjusted until
control glass suspended
Unknown is then added to see if
it floats or sinks
1. Flotation test in density column
2. Theory of Refraction, (Speed)
The speed of light in a vacuum is always the same,
– but when light moves through any other medium it travels
more slowly since it is constantly being absorbed and
reemitted by the atoms in the material.
The ratio of the speed of light in a vacuum to the speed
of light in another substance is defined as the index of
refraction (aka refractive index or n) for the
substance.
Refractometry Link
2. Theory of Refraction, (Speed)
2. Theory of Refraction, (Angular)
Light crossing from any transparent
medium into another in which it has
a different speed, is refracted
– i.e. bent from its original path
(except when the direction of travel is
perpendicular to the boundary).
In the case shown, the speed of
light in medium A is greater than
the speed of light in medium B.
refractometry
2. Theory of Refraction, (Angular)
Refraction Notation
Since the index of refraction depends on both the
1. temperature of the sample
2. the wavelength of light used these are both indicated
when reporting the refractive index:
•
•
•
italicized n denotes refractive index
superscript indicates the temperature in degrees Celsius
subscript denotes the wavelength of light
–
(in this case the D indicates the sodium D line at 589 nm).
Refractive Index
Ratio of speeds in a vacuum vs. a medium
– At a specific temperature
– And Wavelength Frequency
V of light in Vacuum
V of light in medium
Refractive Index
Water at 25C =1.333 (1.333 times
faster in a vacuum then in water at that
temp.
Dependent on temperature and
wavelength frequency
– Sodium D light: STANDARD wavelength
– 589.3 nanometers
Theory of Refraction
Temperature dependence of refractive index for
Sucrose.
Theory of Refraction
Table 1. Temperature dependence of refractive
index for selected substances.
Substance
Isopropanol
Acetone
Ethyl
Acetate
Water
1.3802 1.3772 1.3749
1.3616 1.3588 1.3560
1.3747 1.3742 1.3700
1.3334 1.3330 1.3325
Jell-O (Refractive Index) Lab Demo
Objective:
Use gelatin as a smoked lens, to view total
internal reflection and as a color filter.
Jell-O (Refractive Index)
Lab Demo
Wall Demo
Refractometer
Refractometer
Different Refractometers
Salinity Refractometer: Salinity vs Specific gravity
Brix Refractometer: Sugar content
Immersion Method of Glass
Identification
Immersion method:
– Glass put into liquid
– RI of liquid adjusted by temperature until a match point is
reached.
• Point when Becke line disappears because both liquid
and glass have same RI.
Becke line: a bright halo that is observed near the
border of a particle immersed in a liquid of a
different RI
Becke line:
nglass >nmedium
nglass < nmedium
nmedium = 1.525
nglass = 1.60
nmedium = 1.525
nglass = 1.34
Becke Lines:
3. Glass Refractive Index
Measurement (GRIM 3)
GRIM3 can process glass fragments as small as 50
microns obtained from scenes-of-crime.
Phase contrast optics and a Mettler hotstage, for
temperature control.
Varying temperature to alter the refractive index of a
calibrated oil, the RI of an immersed fragment of glass
can be determined at the point of null refraction, the
point at which the refractive indices of glass and oil
match
http://www.fosterfreeman.com/index.html
3. Glass Refractive Index
Measurement (GRIM 3)
What is the refractive index of the
Unknown Glass Sample?
What would You would need?
Standards????
How Can this be accomplished?
Procedure????
Objective: To Identify the numeric refractive
index of varying Liquid Standards.
Experiment #1 Using the Jell-O
Activity:
Mathematically Develop your
standards.
Experiment #2 Using the Immersion
Method Identification:
Develop your standards
Class Unknown Results?????
Refractive index at 20°C
Baby oil: 1.45
UNKNOWN #2 PYREX
Canola oil: 1.465-1.467
Olive oil: 1.467-1.4705
Soybean oil: 1.470-1.472
Grape Seed Oil: 1.471-1.478, UNKOWN #4 frame
Castor Oil: 1.4750 - 1.4850
UNKNOWN #1 AQUARIUM
Corn Oil: 1.4735 - 1.4785
Xylene: 1.505
Clove Oil: 1.543
Unknowns?????
Refractive index at 20°C
Baby oil: 1.45
UNKNOWN #2 PYREX
Canola oil: 1.465-1.467
Olive oil: 1.467-1.4705
Soybean oil: 1.470-1.472
Grape Seed Oil: 1.471-1.478, UNKOWN #4 frame
Castor Oil: 1.4750 - 1.4850
UNKNOWN #1 AQUARIUM
Corn Oil: 1.4735 - 1.4785
Xylene: 1.505
Clove Oil: 1.543
Glass Samples
#1 picture frame glass: 1.48, 1.47, 1.48, 1.36
#2 Fish Tank: 1.50, 1.466, 1.45, 1.48, 1.45
#4 Beaker Pyrex : 1.471, 1.48, 1.47
#1 picture frame glass: 1.47,
– Grape Seed, Soybean
#2 Fish Tank: 1.50, 1.466, 1.45, 1.48, 1.45
#4 Beaker Pyrex : 1.471, 1.48, 1.47
Class Results “Glass Samples”
#1 picture frame glass:
#2 Fish Tank
#4 Beaker Pyex
#4 Beaker non-Pyrex, Bottle
Soda-lime glass
Soda (NaCo3)
Lime (CaO)
Windows
Bottles
Pyrex Borosilicates use Boron oxide, Test tubes
Can with stand
HIGH heats
Tempered Glass: Rapid heating and
cooling does not
shatter
Headlights
Shower doors
Side + rear
windows
Laminated Glass Plastic or Glass and Windshields
glues and sandwich
Dats It
Remember Slides
Refractive Index
Transparent solids immersed in a liquid
having a similar RI, light will not be
refracted as it passes from liquidsolid.
Reason why the eye unable to distinguis
between the solidliquid boundary.
Solids are crystalline
Crystalline solids: have definite geometric
forms because of the orderly arrangement of
particles (Atoms).
• Relative location/arrangement of atoms
repeats
Atoms: smallest unit of an element
No!! Amorphous Solids
Amorphous solids: atoms or molecule
are arranged RANDOMLY
NO regular order to the atoms
– Glass
What is the refractive index of
Sodium Chloride (NaCl)?
Crystalline solids
Exhibits double refraction (double imagery
produced)
Calcite, RI=1.486 and 1.658
The difference 0.172 is known as
birefringence. Most CALCITE
Dispersion: separation of light into its
component wavelengths